Brushless direct current (DC) motors typically include electronic circuitry that energizes and de-energizes electric coils (windings) in the motor in order to make the rotor spin. Brushless DC motors are commonly used to drive cooling fans in electronic devices such as personal computers (PCs). A typical brushless DC motor used in a PC is packaged in such a way that only two terminals are accessible: a positive power supply terminal VS and a ground terminal GND (also referred to as a positive and a negative rail, respectively). A third terminal which provides a signal that indicates the speed of the motor is sometimes accessible as well.
Cooling fans driven by brushless DC motors have traditionally been run at full speed at all times since this is the simplest implementation. In a typical PC, this is accomplished by simply connecting the GND terminal to a power supply ground, and the VS terminal to the computer's +12 Volt or +5 Volt power supply. This is an inefficient scheme, however, since most electronic devices only require maximum cooling power for short periods of time and at random intervals. Running the fan constantly at full speed wastes energy and generates unnecessary noise.
A recent trend is to run the fan motor at different speeds depending on the cooling demand. One way to accomplish this is to drive the motor with a variable voltage power supply. This is sometimes referred to as linear fan speed control. Linear fan speed control, however, can be difficult and expensive to implement because it requires a variable voltage power supply. There are also problems such as those associated with fact that most 12 Volt fans must initially be driven with at least 6 to 8 Volts to overcome the initial resistance to rotation.
Another solution involves the use of pulse width modulation (PWM). In a PWM scheme, the power supply to the motor is repetitively turned on and off at a fixed frequency but variable duty cycle. When the power supply signal has a relatively low duty cycle, for example 25 percent (that is, the power supply is on 25 percent of the time and off 75 percent of the time), the motor to turns at a relatively slow speed. Increasing the duty cycle causes the motor to spin faster. Full power is achieved by leaving the power supply signal on at all times, i.e., 100 percent duty cycle.